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Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction
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Ceramics International 35 (2009) 649–652 www.elsevier.com/locate/ceramint
Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction Phathaitep Raksa a, Atcharawan Gardchareon a, Torranin Chairuangsri b, Pongsri Mangkorntong a, Nikorn Mangkorntong a, Supab Choopun a,* b
a Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
Received 28 September 2007; received in revised form 29 October 2007; accepted 26 January 2008 Available online 1 May 2008
Abstract The ethanol sensing properties of CuO nanowires prepared by oxidation reaction of copper plate have been examined. The characterization of CuO nanowires by FE-SEM, EDS, and TEM revealed diameters of 100–400 nm and a monoclinic structure with a growth direction along h1 1 0i direction. The ethanol sensing characteristics of CuO nanowires were studied at ethanol concentrations of 100–1000 ppm and working temperatures of 200–280 8C. An increase of resistance was observed under an ethanol vapor atmosphere due to the p-type semi-conducting property of CuO. It was found that the sensitivity, the response and the recovery time depended on the working temperatures and also ethanol concentration. The sensor exhibited the optimum sensitivity of 1.5 to ethanol vapor concentration of 1000 ppm at the working temperature of 240 8C with a response and recovery time of 110 and 120 s, respectively. # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Electron microscopy; E. Sensors
1. Introduction Copper oxide (CuO) is a metal-oxide semiconductor with a narrow band gap of 1.2 eV and is one of the few metal-oxide semiconductors that exhibit p-type conductivity. Also, it is an important building block of most high-temperature superconductors. Recently, nanostructures of semiconductors have caught attention due to the unique property of having a huge surface-tovolume ratio which is expected to enhance the performance of the devices based on semiconductor nanostructures. CuO nanostructures can be synthesized by various growth techniques such as thermal evaporation, thermal decomposition, sol–gel, and oxidation reaction [1–8]. An oxidation reaction technique is one of the more practical and simple ways of synthesis [8]. CuO can be used for a wide range of applications such as photoconductive, photothermal, catalysis and gas sensor [9–13]. For gas sensor application, it is interesting to investigate the effect of CuO nanostructure, which has a huge surface-tovolume ratio, on gas sensing properties. Thus, in this work, the * Corresponding author. Tel.: +66 53 943 375; fax: +66 53 357 511. E-mail address: [email protected] (S. Choopun).
ethanol sensing properties of CuO nanowires prepared by oxidation reaction are examined. The ethanol sensing properties were studied for various ethanol concentrations and working temperatures. 2. Experimental method The commercial grade copper plate with thickness of 0.1 mm was cut and cleaned by alcohol in an ultrasonic bath for 2 min and dried at room temperature. The copper plate was loaded into a center of a tube furnace at 600 8C at normal atmosphere for oxidation reaction. The oxidation reaction [8] follows in Eqs. (1) and (2). 4Cu þ O2 ! 2Cu2 O
(1)
2Cu2 O þ O2 ! 4CuO
(2)
After oxidation reaction for 6 h, the copper plate was taken out of the furnace and immediately cooled down in air. The colour of the copper plate changed to black after oxidation reaction. It has been reported that the colour of CuO, Cu2O and Cu are black, red and copper, respectively [8]. Therefore, the black product that
0272-8842/$34.00 # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2008.01.028
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P. Raksa et al. / Ceramics International 35 (2009) 649–652
Fig. 1. Schematic diagram of a sensor fabricated from CuO nanowires and a layer of CuO.
was separated out from the copper plate after being taken from the furnace was CuO from appeared colour. The CuO was investigated by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6335F) for morphology, energy dispersive spectroscopy (EDS) for chemical composition, and transmission electron microscopy (TEM, JEOL JEM-2010 operating at 200 kV) for the crystal structure. The black product, CuO, was separated from the copper plate for fabrication of ethanol sensor
as shown in the schematic diagram, Fig. 1. It should be noted that the black plate (black product) substance was composed of CuO layered with CuO nanowires. The black plate was cut into a square shape with dimensions of 5 mm 5 mm before silver paint was applied at the diagonal corners of the black square to be silver electrodes, sized 1 mm 1 mm. A heater for the ethanol sensor was made from a nickel-chromium coil and wound around an alumina plate, resistance 80 V, and placed beneath the CuO plate. Finally, copper wires were attached to the silver electrodes for resistance measurement to complete the ethanol sensor. The sensing properties were studied by observing the change of resistance in air and in ethanol ambiences of ethanol concentration of 100, 200, 500 and 1000 ppm, respectively, and at working temperatures of 200–280 8C. The response and the recovery characteristics were monitored and recorded via an interfaced personal computer. 3. Result and discussion 3.1. FE-SEM and TEM analysis Fig. 2(a) and (b) shows FE-SEM top and cross-section view images, respectively of CuO plate (black product) separated
Fig. 2. (a) and (b) show FE-SEM images of top view and cross-section view, respectively, of the CuO plate (black product) separated out from the copper plate after oxidation reaction (c) EDS spectrum of the CuO plate.
P. Raksa et al. / Ceramics International 35 (2009) 649–652
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Table 1 The ethanol sensing properties of CuO nanowires at an ethanol concentration of 1000 ppm, and a working temperature of 200–280 8C Working temperature (8C)
Steady resistance (Rg, V)
Ethanol ambient resistance (Ra, V)
Sensitivity
200 220 240 260 280
5314 4290 3232 2610 2229
7749 6302 4690 2925 2428
1.4 1.5 1.5 1.1 1.1
3.2. Gas sensing properties
from the copper plate after oxidation reaction. The nanowires having a diameter of 100–400 nm and the length of around several micrometers were observed. The result of EDS shown in Fig. 2(c) confirms that the black product is CuO layered with CuO nanowire. A bright-field TEM image of the CuO nanowires is shown in Fig. 3 superimposed with the corresponding SADP. Three wirelike structures with diameter about 100 nm can be observed. The SADP shows a spot pattern indicating the single-crystalline property of the CuO nanowire with monoclinic structure. Also, the spots show line-streaking along [3 0 0] corresponding to the shape factor of the nanowires on the reciprocal lattice node. Trace analysis results suggest that CuO has the growth direction along h1 1 0i direction.
Fig. 4 shows the response and the recovery curves of ethanol sensors based on CuO nanowires being exposed to an ethanol concentration of 1000 ppm and working temperatures of 200– 2808C. At the beginning, the measured resistance was steady in an ambient atmosphere. But when ethanol vapor was injected into the chamber, an increase of resistance was observed; and when ethanol vapor was removed, a decrease of resistance was observed. The increase of resistance under in ethanol vapor atmosphere is due to the p-type conductivity of CuO. It can be clearly seen from Fig. 4 that the characteristic of the sensor depends on the working temperature. It should be noted that the lower resistance of CuO nanowires at higher working temperatures is due to the common, semiconductor property of CuO. The sensitivity, Sg, of the sensor is defined as Rg/Ra where, Rg is the electrical resistance of a sensor in ethanol-air mixed gases; and Ra is the electrical resistance of sensor in air. The sensitivity obtained for the CuO nanowire sensor at different working temperatures at the ethanol concentration of 1000 ppm is listed in Table 1. The highest sensitivity was obtained at the working temperature of 240 8C, suggesting this was the optimum working temperature. In addition, the response time and the recovery time also depended on the working temperature, in the range of 30– 235 s and 60–245 s, respectively. Fig. 5 shows the response and the recovery curves of ethanol sensors based on CuO nanowires when exposed to an ethanol
Fig. 4. The response and recovery curves of the CuO nanowires sensor at an ethanol concentration of 1000 ppm, working temperature of 200–280 8C.
Fig. 5. The response and the recovery curves of the CuO nanowires sensor at ethanol concentration of 100–1000 ppm and working temperature of 240 8C.
Fig. 3. A bright-field TEM image superimposed with the corresponding SADP of the CuO nanowires.
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P. Raksa et al. / Ceramics International 35 (2009) 649–652
concentration of 100–1000 ppm at the working temperature of 240 8C. It was found that the sensitivity only slightly depended on the ethanol concentration. 4. Conclusion The ethanol sensors based on CuO nanowires were successfully fabricated. The CuO nanowires were prepared by an oxidation reaction of copper plate. From FE-SEM, EDS and TEM characterization, CuO nanowires exhibited diameters of 100–400 nm having a monoclinic structure with a growth along h1 1 0i direction. Moreover, the CuO nanowire sensor responded to ethanol vapor, exhibiting the optimum sensitivity of 1.5 to an ethanol vapor concentration of 1000 ppm with a working temperature of 240 8C, the response time of 110 s, and recovery time of 120 s. The CuO nanowires could be explored for gas sensor application. Acknowledgements This work was supported by the Commission of Higher Education (CHE) and Thailand Research Fund (TRF). This work was also supported by the Nanoscience and Nanotechnology Center, Faculty of Science and The Graduate School, Chiang Mai University. The Electron Microscopy Research and Service Center is thanked for the electron microscopy facilities. References [1] L.S. Huang, S.G. Yang, T. Li, B.X. Gu, Y.W. Du, Y.N. Lu, S.Z. Shi, Preparation of large-scale cupric oxide nanowires by thermal evaporation method, Journal of Crystal Growth 260 (2004) 130–135.
[2] W. Wang, Y. Zhan, X. Wang, Y. Lui, C. Zheng, G. Wang, Synthesis and characterization of CuO nanowhiskers by novel one step, solid state reaction in the present of a nonionic surfactant, Materials Research Bulletin 37 (2002) 1093–1100. [3] H. Wang, J.Z. Xu, J.J. Zhu, H.Y. Chen, Preparation of CuO nanoparticles by microwave irradiation, Journal of Crystal Growth 244 (2002) 88–94. [4] J.F. Xu, W. Ji, Z.X. Shen, S.H. Tang, X.R. Ye, D.Z. Jia, X.Q. Xin, Preparation and characterization of CuO nanocrystals, Journal of Solid State Chemistry 147 (1999) 516–519. [5] G. Xu, Y. Liu, G. Xu, G. Wang, Preparation and characterization of CuO nanorods by thermal decomposition of CuC2O4 precursor, Materials Research Bulletin 37 (2002) 2365–2372. [6] T. Yu, X. Zhao, Z.X. Shen, Y.H. Wu, W.H. Su, Investigation of individual CuO nanorods by polarized micro-Raman scattering, Journal of Crystal Growth 268 (2004) 590–595. [7] P. Raksa, S. Kittikunodom, S. Choopun, T. Chairungsri, P. Mangkorntong, N. Mankorntong, CuO Nanowires by Oxidation Reaction, CMU. Journal (Special Issue on Nanotechnology) 4 (2005) 1–5. [8] C.H. Xu, C.H. Woo, S.Q. Shi, Formation of CuO nanowires on Cu foil, Chemical Physics Letters 399 (2004) 62–66. [9] S. Aygun, D. Cann, Hydrogen sensitivity of doped CuO/ZnO heterocontact sensors, Sensors and Actuators B 106 (2005) 837–842. [10] A. Cruccolini, R. Narducci, R. Palombari, Gas adsorption effects on surface conductivity of nonstoichiometric CuO, Sensors and Actuators B 98 (2004) 227–232. [11] M. Frietsch, F. Zudock, J. Goschnick, M. Bruns, CuO catalytic membrane as selectivity trimmer for metal oxide gas sensors, Sensors and Actuators B 65 (2000) 379–381. [12] T. Ishihara, M. Higuchi, T. Takagi, M. Ito, H. Nishiguchi, Y. Takita, Preparation of CuO thin films on porous BaTiO3 by self-assembled multibilayer film formation and application as a CO2 sensor, Journal of Materials Chemistry 8 (9) (1998) 2037–2042. [13] J. Tamaki, K. Shimanoe, Y. Yamada, Y. Yamamoto, N. Miura, N. Yamazoe, Dilute hydrogen sulfide sensing properties of CuO–SnO2 thin film prepared by low-pressure evaporation method, Sensors and Actuators B 49 (1998) 121–125.
Ceramics International 35 (2009) 649–652 www.elsevier.com/locate/ceramint
Ethanol sensing properties of CuO nanowires prepared by an oxidation reaction Phathaitep Raksa a, Atcharawan Gardchareon a, Torranin Chairuangsri b, Pongsri Mangkorntong a, Nikorn Mangkorntong a, Supab Choopun a,* b
a Department of Physics, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand Department of Industrial Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
Received 28 September 2007; received in revised form 29 October 2007; accepted 26 January 2008 Available online 1 May 2008
Abstract The ethanol sensing properties of CuO nanowires prepared by oxidation reaction of copper plate have been examined. The characterization of CuO nanowires by FE-SEM, EDS, and TEM revealed diameters of 100–400 nm and a monoclinic structure with a growth direction along h1 1 0i direction. The ethanol sensing characteristics of CuO nanowires were studied at ethanol concentrations of 100–1000 ppm and working temperatures of 200–280 8C. An increase of resistance was observed under an ethanol vapor atmosphere due to the p-type semi-conducting property of CuO. It was found that the sensitivity, the response and the recovery time depended on the working temperatures and also ethanol concentration. The sensor exhibited the optimum sensitivity of 1.5 to ethanol vapor concentration of 1000 ppm at the working temperature of 240 8C with a response and recovery time of 110 and 120 s, respectively. # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: B. Electron microscopy; E. Sensors
1. Introduction Copper oxide (CuO) is a metal-oxide semiconductor with a narrow band gap of 1.2 eV and is one of the few metal-oxide semiconductors that exhibit p-type conductivity. Also, it is an important building block of most high-temperature superconductors. Recently, nanostructures of semiconductors have caught attention due to the unique property of having a huge surface-tovolume ratio which is expected to enhance the performance of the devices based on semiconductor nanostructures. CuO nanostructures can be synthesized by various growth techniques such as thermal evaporation, thermal decomposition, sol–gel, and oxidation reaction [1–8]. An oxidation reaction technique is one of the more practical and simple ways of synthesis [8]. CuO can be used for a wide range of applications such as photoconductive, photothermal, catalysis and gas sensor [9–13]. For gas sensor application, it is interesting to investigate the effect of CuO nanostructure, which has a huge surface-tovolume ratio, on gas sensing properties. Thus, in this work, the * Corresponding author. Tel.: +66 53 943 375; fax: +66 53 357 511. E-mail address: [email protected] (S. Choopun).
ethanol sensing properties of CuO nanowires prepared by oxidation reaction are examined. The ethanol sensing properties were studied for various ethanol concentrations and working temperatures. 2. Experimental method The commercial grade copper plate with thickness of 0.1 mm was cut and cleaned by alcohol in an ultrasonic bath for 2 min and dried at room temperature. The copper plate was loaded into a center of a tube furnace at 600 8C at normal atmosphere for oxidation reaction. The oxidation reaction [8] follows in Eqs. (1) and (2). 4Cu þ O2 ! 2Cu2 O
(1)
2Cu2 O þ O2 ! 4CuO
(2)
After oxidation reaction for 6 h, the copper plate was taken out of the furnace and immediately cooled down in air. The colour of the copper plate changed to black after oxidation reaction. It has been reported that the colour of CuO, Cu2O and Cu are black, red and copper, respectively [8]. Therefore, the black product that
0272-8842/$34.00 # 2008 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2008.01.028
650
P. Raksa et al. / Ceramics International 35 (2009) 649–652
Fig. 1. Schematic diagram of a sensor fabricated from CuO nanowires and a layer of CuO.
was separated out from the copper plate after being taken from the furnace was CuO from appeared colour. The CuO was investigated by field emission scanning electron microscopy (FE-SEM, JEOL JSM-6335F) for morphology, energy dispersive spectroscopy (EDS) for chemical composition, and transmission electron microscopy (TEM, JEOL JEM-2010 operating at 200 kV) for the crystal structure. The black product, CuO, was separated from the copper plate for fabrication of ethanol sensor
as shown in the schematic diagram, Fig. 1. It should be noted that the black plate (black product) substance was composed of CuO layered with CuO nanowires. The black plate was cut into a square shape with dimensions of 5 mm 5 mm before silver paint was applied at the diagonal corners of the black square to be silver electrodes, sized 1 mm 1 mm. A heater for the ethanol sensor was made from a nickel-chromium coil and wound around an alumina plate, resistance 80 V, and placed beneath the CuO plate. Finally, copper wires were attached to the silver electrodes for resistance measurement to complete the ethanol sensor. The sensing properties were studied by observing the change of resistance in air and in ethanol ambiences of ethanol concentration of 100, 200, 500 and 1000 ppm, respectively, and at working temperatures of 200–280 8C. The response and the recovery characteristics were monitored and recorded via an interfaced personal computer. 3. Result and discussion 3.1. FE-SEM and TEM analysis Fig. 2(a) and (b) shows FE-SEM top and cross-section view images, respectively of CuO plate (black product) separated
Fig. 2. (a) and (b) show FE-SEM images of top view and cross-section view, respectively, of the CuO plate (black product) separated out from the copper plate after oxidation reaction (c) EDS spectrum of the CuO plate.
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Table 1 The ethanol sensing properties of CuO nanowires at an ethanol concentration of 1000 ppm, and a working temperature of 200–280 8C Working temperature (8C)
Steady resistance (Rg, V)
Ethanol ambient resistance (Ra, V)
Sensitivity
200 220 240 260 280
5314 4290 3232 2610 2229
7749 6302 4690 2925 2428
1.4 1.5 1.5 1.1 1.1
3.2. Gas sensing properties
from the copper plate after oxidation reaction. The nanowires having a diameter of 100–400 nm and the length of around several micrometers were observed. The result of EDS shown in Fig. 2(c) confirms that the black product is CuO layered with CuO nanowire. A bright-field TEM image of the CuO nanowires is shown in Fig. 3 superimposed with the corresponding SADP. Three wirelike structures with diameter about 100 nm can be observed. The SADP shows a spot pattern indicating the single-crystalline property of the CuO nanowire with monoclinic structure. Also, the spots show line-streaking along [3 0 0] corresponding to the shape factor of the nanowires on the reciprocal lattice node. Trace analysis results suggest that CuO has the growth direction along h1 1 0i direction.
Fig. 4 shows the response and the recovery curves of ethanol sensors based on CuO nanowires being exposed to an ethanol concentration of 1000 ppm and working temperatures of 200– 2808C. At the beginning, the measured resistance was steady in an ambient atmosphere. But when ethanol vapor was injected into the chamber, an increase of resistance was observed; and when ethanol vapor was removed, a decrease of resistance was observed. The increase of resistance under in ethanol vapor atmosphere is due to the p-type conductivity of CuO. It can be clearly seen from Fig. 4 that the characteristic of the sensor depends on the working temperature. It should be noted that the lower resistance of CuO nanowires at higher working temperatures is due to the common, semiconductor property of CuO. The sensitivity, Sg, of the sensor is defined as Rg/Ra where, Rg is the electrical resistance of a sensor in ethanol-air mixed gases; and Ra is the electrical resistance of sensor in air. The sensitivity obtained for the CuO nanowire sensor at different working temperatures at the ethanol concentration of 1000 ppm is listed in Table 1. The highest sensitivity was obtained at the working temperature of 240 8C, suggesting this was the optimum working temperature. In addition, the response time and the recovery time also depended on the working temperature, in the range of 30– 235 s and 60–245 s, respectively. Fig. 5 shows the response and the recovery curves of ethanol sensors based on CuO nanowires when exposed to an ethanol
Fig. 4. The response and recovery curves of the CuO nanowires sensor at an ethanol concentration of 1000 ppm, working temperature of 200–280 8C.
Fig. 5. The response and the recovery curves of the CuO nanowires sensor at ethanol concentration of 100–1000 ppm and working temperature of 240 8C.
Fig. 3. A bright-field TEM image superimposed with the corresponding SADP of the CuO nanowires.
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P. Raksa et al. / Ceramics International 35 (2009) 649–652
concentration of 100–1000 ppm at the working temperature of 240 8C. It was found that the sensitivity only slightly depended on the ethanol concentration. 4. Conclusion The ethanol sensors based on CuO nanowires were successfully fabricated. The CuO nanowires were prepared by an oxidation reaction of copper plate. From FE-SEM, EDS and TEM characterization, CuO nanowires exhibited diameters of 100–400 nm having a monoclinic structure with a growth along h1 1 0i direction. Moreover, the CuO nanowire sensor responded to ethanol vapor, exhibiting the optimum sensitivity of 1.5 to an ethanol vapor concentration of 1000 ppm with a working temperature of 240 8C, the response time of 110 s, and recovery time of 120 s. The CuO nanowires could be explored for gas sensor application. Acknowledgements This work was supported by the Commission of Higher Education (CHE) and Thailand Research Fund (TRF). This work was also supported by the Nanoscience and Nanotechnology Center, Faculty of Science and The Graduate School, Chiang Mai University. The Electron Microscopy Research and Service Center is thanked for the electron microscopy facilities. References [1] L.S. Huang, S.G. Yang, T. Li, B.X. Gu, Y.W. Du, Y.N. Lu, S.Z. Shi, Preparation of large-scale cupric oxide nanowires by thermal evaporation method, Journal of Crystal Growth 260 (2004) 130–135.
[2] W. Wang, Y. Zhan, X. Wang, Y. Lui, C. Zheng, G. Wang, Synthesis and characterization of CuO nanowhiskers by novel one step, solid state reaction in the present of a nonionic surfactant, Materials Research Bulletin 37 (2002) 1093–1100. [3] H. Wang, J.Z. Xu, J.J. Zhu, H.Y. Chen, Preparation of CuO nanoparticles by microwave irradiation, Journal of Crystal Growth 244 (2002) 88–94. [4] J.F. Xu, W. Ji, Z.X. Shen, S.H. Tang, X.R. Ye, D.Z. Jia, X.Q. Xin, Preparation and characterization of CuO nanocrystals, Journal of Solid State Chemistry 147 (1999) 516–519. [5] G. Xu, Y. Liu, G. Xu, G. Wang, Preparation and characterization of CuO nanorods by thermal decomposition of CuC2O4 precursor, Materials Research Bulletin 37 (2002) 2365–2372. [6] T. Yu, X. Zhao, Z.X. Shen, Y.H. Wu, W.H. Su, Investigation of individual CuO nanorods by polarized micro-Raman scattering, Journal of Crystal Growth 268 (2004) 590–595. [7] P. Raksa, S. Kittikunodom, S. Choopun, T. Chairungsri, P. Mangkorntong, N. Mankorntong, CuO Nanowires by Oxidation Reaction, CMU. Journal (Special Issue on Nanotechnology) 4 (2005) 1–5. [8] C.H. Xu, C.H. Woo, S.Q. Shi, Formation of CuO nanowires on Cu foil, Chemical Physics Letters 399 (2004) 62–66. [9] S. Aygun, D. Cann, Hydrogen sensitivity of doped CuO/ZnO heterocontact sensors, Sensors and Actuators B 106 (2005) 837–842. [10] A. Cruccolini, R. Narducci, R. Palombari, Gas adsorption effects on surface conductivity of nonstoichiometric CuO, Sensors and Actuators B 98 (2004) 227–232. [11] M. Frietsch, F. Zudock, J. Goschnick, M. Bruns, CuO catalytic membrane as selectivity trimmer for metal oxide gas sensors, Sensors and Actuators B 65 (2000) 379–381. [12] T. Ishihara, M. Higuchi, T. Takagi, M. Ito, H. Nishiguchi, Y. Takita, Preparation of CuO thin films on porous BaTiO3 by self-assembled multibilayer film formation and application as a CO2 sensor, Journal of Materials Chemistry 8 (9) (1998) 2037–2042. [13] J. Tamaki, K. Shimanoe, Y. Yamada, Y. Yamamoto, N. Miura, N. Yamazoe, Dilute hydrogen sulfide sensing properties of CuO–SnO2 thin film prepared by low-pressure evaporation method, Sensors and Actuators B 49 (1998) 121–125.